Giant jellyfish with bodies the size of washing machines have turned up off the central coast of Japan in their largest numbers for decades.

The jellyfish, which are three feet wide, weigh as much as 330 pounds.

They began appearing in the Sea of Japan in mid-August, near the town of Echizen.

The species, identified as stomolophus nomurai, were first found in the Sea of Japan in 1920, said Toru Yasuda, a marine biologist formerly at Fukui University.

But fishermen haven't caught the jellyfish in such large numbers since 1958, he said.

"We don't know what caused it (the population surge)," Yasuda said. "One possible reason is the abnormally warm water temperatures."

In September and October, several fishermen reported more than a thousand jellyfish with bodies the size of washing machines trapped in their nets at a time.

The jellyfish can grow as long as 15 feet, but the poison in their tentacles isn't lethal to humans.

Many fishermen have complained that the jellyfish have cut their hauls by more than half, and discoloured or sickened other fish and shrimp in the catch that have been stung by their tentacles, the official said.